Why does a locksmith need me to remove my entire car door?

So I bought a 1992 fleetwood. And for some unknown reason the keys to the doors are not the same as the key to the ignition. And when I bought the car, the owner told me flat out he lost the keys to the door. I thought thats fine, I'll call a locksmith to maybe make me a new one.

Turns out, there's 2 issues. Every car-specific locksmiths in my area only work on cars after 2005 for some reason. And every regular locksmith like for normal doors and stuff told me I have to physically remove the entire lock cylinder from my car and bring it to them for them to make me a new key. Now I dont know much about lock smithing, but can I ask why? The keyhole is there, can't they figure it out somehow with the keyhole? Why do I need to rip apart my entire door and bring them the lock cylinder for a new key?